This invention relates to a process of producing a gas which can be substituted for natural gas and which is produced from a raw gas that contains hydrogen and carbon oxides and has been produced by a gasification of coal, tar or heavy residual oils under superatmospheric pressure, in which process the raw gas is cooled, purified to remove catalyst poison, particularly sulfur compounds, and then subjected to methanation in at least two stages in contact with nickel catalysts under pressures of 5-100 bars and at temperatures in the range of 200.degree.-500.degree. C.
It is known that coal, tar, and heavy residual oil can be degasified with water vapor and oxygen under superatmospheric pressure and at elevated temperatures to produce a raw gas which contains carbon oxides in excess of its hydrogen content. The raw gases produced by the gasification of coal with water vapor and oxygen under a pressure of 20-80 bars generally have a high CO.sub.2 content (28-32% by volume) and CO content (15-20% by volume) and a relatively low H.sub.2 content (35-44% by volume). The gasification of tars and residual oils is carried out at higher temperatures of about 1100.degree.-1500 .degree. C than the gasification of coal and results in a raw gas which contains 3-6% by volume CO.sub.2, 46-50% by volume CO, and 40-48% by volume H.sub.2. Heavy residual oils are hydrocarbons which boil above 250 .degree. C.
The raw gases which have been purified mainly to remove all sulfur compounds may be converted by known single- or multi-stage methanation processes with the aid of nickel catalysts to form gases which contain at least 90% by volume methane (calculated as CO.sub.2 -free gas) and which can be substituted for or mixed with natural gas. The hydrogen content of these gases should not exceed 2% by volume and hydrogen contents below 1% by volume are often required. The gas contains in most cases inert components consisting of nitrogen, argon, and residual carbon dioxide.
By the methanation, the oxides of carbon are catalytically reacted with hydrogen to form methane and water vapor by the following exothermic reactions: EQU CO + 3 H.sub.2 = CH.sub.4 = H.sub.2 O EQU co.sub.2 + 4 h.sub.2 = 3 ch.sub.4 + 2h.sub.2 o.
from thermodynamic laws it is known that the reaction temperatures should be as low as possible in order to produce a gas having a high methane content and low contents of residual carbon oxide and hydrogen. For this reason a large proportion of the water vapor formed in the preceding reaction stages must be removed from the gas before the same is passed through the last catalyst bed so that the formation of additional methane at a given temperature is promoted.
In a known process the scrubbed raw gas is first reacted in a so-called recycling methanation stage, which is left by the gas at a relatively high temperature. In this stage the activity of the nickel catalyst at temperatures above about 280.degree. C is utilized. Part of the product gas from this methanation stage is cooled in a waste heat boiler and is then recycled to the inlet of this stage in order to dilute the feed gas. This recycling methanation stage may comprise a plurality of stages in an arrangement in which the raw gas is divided into a plurality of partial streams and passed through reactors connected in series, the gas which leaves the first reactor serving as a diluent gas for the second reactor and part of the product gas from the last reactor being recycled to serve as diluent gas for the first reactor.
Owing to the equilibrium which results at the relatively high temperature, the product gas which has formed in the recycling methanation stage still contains a considerable proportion of hydrogen, which can be reacted with additional carbon oxides to form methane only at lower temperatures. For this reason it is known to cool the product gas from the recycling methanation stage and to subject the cooled gas to a first aftermethanation in a so-called wet aftermethanation stage without a previous removal of water vapor from the gas. Because the wet methanation does not result in a product gas having the desired methane content, it is also usual to cool the resulting gas, to remove by condensation a major part of the water vapor contained in the gas, then to reheat the gas and to pass it again over nickel catalysts. This so-called dry aftermethanation stage produces the desired gas which can be substituted for natural gas when surplus CO.sub.2 gas been scrubbed out.
In some cases the wet aftermethanation stage is omitted and the product gas withdrawn from the recycling stage is directly cooled, water vapor is condensed out of the gas, and the latter is then subjected to the dry aftermethanation. This practice results in a product gas which has a lower methane content and enables only a lower recovery of heat of reaction which has been liberated. For this reason, the overall thermal efficiency of the plant is lower than with the process described first. Besides, owing to the low water vapor content and the relatively high temperature of the gas leaving the aftermethanation stage result the risk of a formation of carbon black by the Boudouard reaction is much higher than in the process in which a wet aftermethanation and a dry aftermethanation are carried out in succession.
This invention simplifies methanation in a process of the kind defined first hereinbefore and omits a removal of water vapor by condensation before the last methanation stage. At the same time the heat of reaction should be utilized to a higher degree. This is accomplished according to the invention in that the product gas from the preceding methanation stage is reacted in the last methanation stage in contact with a catalyst which is indirectly cooled by a gas flowing in a countercurrent to the gas to be reacted. The cooling with gas in the methanation reactor permits of a utilization of the catalyst at reaction temperatures which are as low as possible whereas adiabatic methanation stages are capable of a stable operation only at higher temperatures. Because even the last stage operates under wet methanation conditions, the residual gas may contain slightly more water vapor than where a dry methanation is performed, whereas the hydrogen content still remains below the upper limit to be observed.